Test Cross: Definition, Meaning, Examples, Synonyms, Genetics

A test cross is a genetic technique that is used to determine the unknown genotype of an organism that shows dominant trait. Since both homozygous dominant and heterozygous individuals show the same phenotype, crossing the individual with a homozygous recessive reveals the actual genetic makeup. This is widely used in genetics to identify carriers and confirm patterns of inheritance.

This Story also Contains

1. What Is a Test Cross?
2. Basics Of Genetics
3. Mendelian Genetics Overview
4. How To Perform A Test Cross
5. Examples Of Test Crosses
6. Recommended video for "Test Cross"
7. MCQs on Test Cross

Gregor Mendel used pea plants in his experiments to study test cross. His observations of offsprings laid the laws of inheritance. In the contemporary world, test crosses are important tools in genetics, helping scientists to discover variety in genes, linkage and recombination as well as hereditary patterns in inheritance in different generations.

What Is a Test Cross?

A test cross is a genetic method that is used to determine the genotype of an organism that shows a dominant trait. This is done by crossing it with an organism that is homozygous recessive for the same trait. The result helps to identify whether the dominant organism contains homozygous dominant or heterozygous alleles.

Gregor Mendel did similar experiments on pea plants using test cross. He studied the patterns of inheritance to determine the degree of dominance and found out the basic laws of inheritance.

Basics Of Genetics

Genetics is the study of how traits are passed to successive generations through genes, which are the simplest hereditary factors. Genes are made up of DNA and they determine certain characteristics such as eye color, height, or flower position in plants. Every gene is present in two forms called alleles and they are responsible for variations intraits.

A dominant allele is the one that expresses itself even when only one copy is present (). For example, in pea plants, the allele for purple flower (P) is dominant over the allele for white flowers (p). So, both PP (homozygous dominant) and Pp (heterozygous) plants will have purple flowers. While pp (homozygous recessive) plants will have white flowers. It is important to understand the difference between homozygous and heterozygous genotypes to predict how traits are inherited. These basic genetic principles form the foundation for exploring complex patterns of inheritance and gene distribution among populations.

Mendelian Genetics Overview

Mendel’s experiment with pea plants laid the foundation for modern genetics. He observed patterns of inheritance over several generations and put forward rules for how traits are inherited. His work introduced the concept of alleles and how they contribute to genetic variation. These findings became the basis of Mendelian genetics which explained how traits are passed from one generation to the next.

Mendel’s Laws Of Inheritance

Mendel’s laws of inheritance describe how traits are passed from parents to offsprings through genes. These laws explain how dominant and recessive traits appear. He formulated the three laws of inheritance based on his experiments:

1. Mendel’s Law of Dominance: According to this law, when two different alleles are present for a trait, one is dominant, its trait is expressed. While, the other is recessive and its expression remains masked. The dominant allele masks the effect of the recessive one when both alleles are present.

Explanation of Mendel’s Law of Dominance: Mendel discovered this through his monohybris cross in pea plants. For example, when he crossed tall (TT) and dwarf (tt), all the offspring in the first generation were tall (Tt). This showed that the tall trait (T) was dominant over the dwarf trait (t). Even when both the alleles were present, only the dominant trait appeared in the phenotype.

2. Mendel’s Law of Segregation: For each gene, there are two alleles. During gamete formation, there is a random shuffling of these alleles and gametes are formed containing only one allele for each gene. This explains how offspring can inherit different combinations of traits and leads to genetic diversity in them.

Explanation of Mendel’s Law of Segregation: This law means that every individual possesses two alleles of a characteristic; one from the mother and the other from the father. In gametes, these alleles are distributed randomly and each gamete contains only one allele of each phenotype. Mendel observed a 1:2:1 genotypic ratio in monohybrid cross.

3. Mendel’s Law of Independent Assortment: A gene that is present on one chromosome can be inherited independently of the gene present on the other chromosome during gametes formation. This law holds that no two characters can be inherited independently of each other if they are located on the same chromosome.

Explanation of Mendel’s Law of Independent Assortment: According to this law, the act of alleles of two different genes getting separated is independent of each other and this process takes place only when such genes have their seats on two different chromosomes or are situated in different parts of the same chromosome, if at all there is such a length. This principle explains the inheritance patterns of dihybrid cross, where traits assort independently. The offspring are generated in a 3:3:1 phenotypic ratio.

Role In Determining Genotype

A test cross is useful in finding out the genotype of an organism showing a dominant trait. If all offspring show the dominant trait, the parent is most likely homozygous dominant. But if some offspring show the recessive trait, it means that the parent is heterozygous. This method helps to find recessive genes and it is used in plant and animal breeding to select for desired traits with known genetic makeup.

Differentiating Between Homozygous Dominant And Heterozygous

This cross also helps to see whether the condition is a homozygous dominant or a heterozygous. Hence, it helps to understand the genetic make-up of an organism in terms of its genes and their inheritance.

How To Perform A Test Cross

A test cross is used to determine the genotype of an organism showing dominant trait. It is done by crossing a dominant organism with a homozygous recessive organism of the same trait. By observing the traits, one can determine whether the dominant organism is homozygous or heterozygous for that gene.

Steps Involved In A Test Cross

Performing a test cross involves specific steps to determine the genotype of an organism displaying a dominant phenotype:

1. Select the Organism: Select one organism having the dominant phenotype whose genotype you wish to know. For example, if you are interested in knowing the genotype of a plant with purple flowers – PP or Pp, this plant is your organism.

2. Choose a Recessive Homozygous Organism: Choose a second organism that is homozygous recessive for the same trait, an organism which is pp. This organism will be placed on the other side of the tester in the cross.

3. Cross the Organisms: Breed the organism showing dominant trait (e.g., PP or Pp) with one that has recessive trait (pp). This can be done by controlled pollination or by controlled mating depending on the species.

4. Observe the Offspring: Let the cross reproduce and observe the outcome of the new characters that are developed. Note the number and the kind of offspring showing the dominant and the recessive phenotypes.

5. Interpret the Results: When all the characters of the offspring resemble the dominant organism, then the original organism is homozygous dominant (PP). If mating offspring show the result of the recessive genotype, then the original organism is normally a heterozygote (Pp). These outcomes reveal how gene interaction affects the expression of traits in the next generation.

Selecting The Right Organism For A Test Cross

Select organisms with known phenotypes and genotypes to accurately interpret the results. For example, when working with the flowering process of a pea plant, use flowers with distinct traits like large purple (dominant) and small white (recessive) flowers to study inheritance patterns effectively.

Using A Recessive Homozygous Organism In The Cross

The recessive homozygous organism reveals if the dominant parent carries a recessive allele. If any offspring show the recessive trait, it means the dominant parent is heterozygous. This makes it easier to identify the genotype of the parent.

Examples Of Test Crosses

Test cross is used in genetics to identify the genotype of organisms showing dominant traits. They are especially helpful in experiments involving patterns of inheritance in plants and animals. Some common examples of test cross include:

Test Cross In Pea Plants (Mendel’s Experiments)

Mendel performed several experiments involving test crosses in pea plants to develop his law of inheritance. In one experiment he studies flower color by crossing purple flowers (PP or Pp) with white flowers (pp). The plant with the purple flowers signified the organism with the dominant trait and Mendel wished to identify its exact genotype, which was either PP or Pp.

Through the observation of the offspring, all of them had purple flowers in the F1 generation which suggested the parent with purple flowers might have been homozygous dominant (PP). But when he allowed the F1 generation plants to self-pollinate and examined the F2 generation, he observed a 3:1 ratio of purple to white flowers. This supported that F1 plants were actually heterozygous (Pp), carrying both dominant and recessive alleles.

Test Cross In Fruit Flies (Drosophila melanogaster)

In experiments using Drosophila melanogaster, test crosses establish the genotypes present in dominant characteristics. For example, when studying eye colour, a male red-eyed fly (genotype either XᴿY or XᴿX) is crossed a white-eyed female (genotype XʷXʷ ). In this case, the white-eyed fly is homozygous recessive for the eye colour gene, XʷXʷ.

The progeny of such a cross will reveal if the red-eyed parent is homozygous dominant (XᴿX XᴿX) or heterozygous (Xʷ Xʷ). If all the children have red eyes, the male is likely a homozygous dominant (XᴿX XᴿX). However, if some offspring have white eyes it means the red-eyed parent has the dominant gene only for eye colour (XᴿX).

Recommended video for "Test Cross"

MCQs on Test Cross

Q1. The genotype of a plant showing the dominant phenotype can be determined by

1. Back Cross

2. Test Cross

3. Dihybrid Cross

4. Pedigree Analysis

Correct Answer: 2) Test Cross

Explanation:

A test cross is a genetic cross between an individual showing the dominant phenotype (but with an unknown genotype) and an individual with the homozygous recessive genotype. The purpose of the test cross is to determine the genotype of the individual with the dominant phenotype.

By crossing the individual of an unknown genotype with a known homozygous recessive individual, the phenotype of the offspring can reveal the genotype of the parent with the dominant phenotype. If the individual with the dominant phenotype is homozygous dominant (genotype AA), all the offspring in the test cross will show the dominant phenotype. However, if the individual with the dominant phenotype is heterozygous (genotype Aa), approximately half of the offspring will show the dominant phenotype, and the other half will show the recessive phenotype.

Hence, the correct answer is Option 2) Test cross.

Q2. In a plant, black seed color (BB/Bb) is dominant over white seed color (bb). In order to find out the genotype of the black seed plant, with which of the following genotypes will you cross it?

1. BB

2. Bb

3. bb

4. BB/Bb

Correct Answer: 3) bb

Explanation:

In a test cross, an organism with an unknown genotype is mated to an organism with a known genotype in order to ascertain the genotype of the unknown organism. The test cross is said to have been created by Gregor Mendel. To determine the genotype of a black seed colour at F2, the black seed from F2 is crossed with the white seed colour. This is called a test cross. To determine the genotype of (BB/Bb) black seed we need to cross them with white seed (bb)

Hence, the correct answer is Option (3) bb.

Q3. A test cross is carried out to:

1. predict whether two traits are linked

2. assess the number of alleles of a gene

3. determine whether two species or varieties will breed successfully

4. determine the genotype of a plant at F2

Correct Answer: 4) determine the genotype of a plant at F2

Explanation:

A test cross is performed to:

1. Establish the genotype of an individual:
This process distinguishes whether an organism exhibiting a dominant trait is homozygous dominant or heterozygous.

2. Examine genetic proportions:
It offers insight into the division of alleles and their autonomous distribution during Mendelian inheritance.

How a Test Cross Operates: An organism with an undetermined genotype is bred with one that is homozygous recessive for the same characteristic.
The subsequent offspring's phenotypes disclose the genotype of the ambiguous parent.

Hence, the correct answer is option 4) determine the genotype of a plant at F2.

Also Read:

• MCQs on Test Cross

• Polygenic Inheritance

• Chromosomal Abnormalities